The present invention relates to a process for providing high purity acetylene gas obtained from dissolved acetylene which is charged in a dissolved acetylene bomb and an apparatus used in the process, a process for purifying acetylene and an apparatus used in the process, a process for purifying the inside of a bomb for dissolved acetylene and a process for charging high purity acetylene into a bomb, and more particularly to a process for purifying the inside of a bomb for high purity dissolved acetylene, which is used as a bomb for storing and supplying high purity dissolved acetylene being employed as a source material used for a functional thin film having a crystal structure such as Si--C structure or diamond cubic, which is used in a high efficiency semi-conductor, an electron device, and the like, and a process for charging high purity acetylene rapidly and safely into a bomb while maintaining the purity.
Conventionally, acetylene has hitherto been employed as a raw material in the field of acetylene welding or in the chemical industry.
Acetylene which is employed as a raw material in the field of acetylene welding or chemical industry is prepared from calcium carbide or petroleum. The crude acetylene such as carbide acetylene which is prepared from calcium carbide or petroleum acetylene which is prepared from petroleum just after the preparation, is purified to remove impurities such as PH.sub.3, H.sub.2 S, NH.sub.3 and AsH.sub.3 which are harmful for health or which deteriorate the mechanical strength of a weld zone when the acetylene is employed for welding and to remove impurities such as O.sub.2, PH.sub.3 and H.sub.2 S which are catalyst poisons in a chemical reaction and to give purified acetylene having a purity of not less than 98% by volume which is prescribed in JIS (Japanese Industrial Standards) K 1901 and JIS K 1902.
For instance, in dissolved acetylene which is prepared from commercially available calcium carbide, acetylene is contained in a content of 98.0 to 99.5% by volume and as the other components, impurities such as vapour of solvent, nitrogen, oxygen and argon which come from air are contained as main impurities and impurities such as NH.sub.3, PH.sub.3, H.sub.2 S, hydrogen and CH.sub.4 which come from calcium carbide are contained as the other impurities. It is reported from Osaka Kankyo Gijutsu Center that as one result of the analysis of dissolved acetylene, the dissolved acetylene contains acetylene of 99.10% by volume, a solvent, i.e., acetone of 0.38% by volume, hydrogen of 0.01% by volume, nitrogen of 0.2844% by volume, oxygen of 0.0756% by volume, CO of less than 0.01% by volume, CO.sub.2 of less than 0.01% by volume and CH.sub.4 of 0.13% by volume.
Commercially available petroleum acetylene contains acetylene in a content of about 99.5% by volume and impurities such as C.sub.2 H.sub.4 and CO.sub.2.
With the progress of technology, recently, new materials have been produced from acetylene. For instance, acetylene has been employed as a source material of polyacetylene which is a conductive organic material or an acetylene derivative which is a photosensitive organic material. Also, acetylene which has high reactivity has been employed as a source material of carbon contained in a carbide having a high purity which is employed as functional materials being called fine ceramics or as source materials for producing a-SiC:H having a high conversion efficiency of solar energy, which is sometimes employed as a window layer of a-Si solar cells.
There is a necessity to strictly control the components of a material in order to give a material having required properties. Therefore, acetylene having a high purity which can be used as a source material has been required.
In a dissolved acetylene bomb, i.e., a bomb for dissolved acetylene, conventionally, calcium silicate having a stable physical property is filled in a form of semi-crystal which is a so-called mass in a content of not more than 10% by volume of the content volume of the dissolved acetylene bomb. The mass in the dissolved acetylene bomb generally has a porosity of not less than 90% by volume. The mass is a porous material which is prepared by filling a dissolved acetylene bomb with a mixture of raw materials comprising calcium silicate as a main component and the like and a prescribed amount of water and heating the bomb in a heating furnace to react the mixture. In the space and on the inside wall of the dissolved acetylene bomb, small amounts of impurity gases such as nitrogen gas, oxygen gas, water vapour and carbon dioxide gas, water, and the like, which are introduced, e.g., when the mass is formed and the bomb is subjected to airtightness test with air having a pressure of about 28 atoms which is 1.1 times that of working pressure, are contained.
In case that the bomb is newly employed as a bomb for acetylene, since it is not preferable that impurity gas, in particular, oxygen gas is contained in the bomb, so-called a process for washing with purified acetylene gas, that is, after purified acetylene is charged into a bomb under a pressure of about 3 Kg/cm.sup.2.G and the bomb is allowed to stand, residual impurity gas or vapour contained in the bomb is liberated with the acetylene gas to purify the inside of the bomb when the dissolved acetylene is gasified and liberated. The cleanliness of the inside of the bomb is determined by analyzing the liberated gases and vapours and measuring the concentration of the acetylene gas. The standard of cleanliness is that the concentration of acetylene gas is not less than 98.0% by volume as mentioned above.
As a process for charging acetylene gas into a bomb in which a solvent which selectively dissolves acetylene is charged, conventionally, a process that acetylene gas is pressured with a compressor and the pressured acetylene gas is charged into the bomb by the difference between the pressure of the pressured acetylene gas and the internal pressure of the bomb.
According to the above process, it takes 6 to 8 hours to charge acetylene gas into a bomb under a pressure of 20 to 25 Kg/cm.sup.2.G to satisfy the legal standard for charging acetylene, that is, the internal pressure of the bomb should be not more than 15.5 Kg/cm.sup.2.G at 15.degree. C. since the temperature of the acetylene is increased by pressing the acetylene at ordinary temperature and the temperature of the acetylene is further increased by the heat of dissolution of the acetylene when the acetylene is dissolved into a solvent in a bomb, and thereby the inside temperature of the bomb is increased and the solubility of acetylene is lowered although the temperature of acetylene is somewhat decreased by sprinkling the bomb with water.
As a process for preparing purified acetylene which is used in new materials, a process for preparing purified acetylene comprising removing a solvent with a low temperature trap or an adsorbent such as silica gel, cooling acetylene to not more than about -150.degree. C. to give solid acetylene with a bath of liquid nitrogen, removing vapours contained in the trap by evacuating the trap and vaporizing impurities contained in the solid acetylene is well-known. The process is suitable for being used in a small scale laboratory but there is a risk for explosive decomposition when the absolute pressure of the trap is not less than 1.4 Kg/cm.sup.2. Therefore, the process is not suitable for safely and economically providing acetylene in an industrial necessary amount.
First of all, the object of the present invention is to provide a process for providing high purity acetylene gas, comprising transporting purified acetylene gas safely in a necessary amount for industrial uses to a place where the acetylene gas is used and preparing high purity acetylene at the place.
Second, the object of the present invention is to provide purified acetylene safely in a necessary amount for industrial uses.
Also, according to a conventional process for purifying the inside of a dissolved acetylene bomb, it is difficult to remove residual impurity gases such as nitrogen gas and oxygen gas contained in the space of the bomb and to reduce the content of impurity gases to an order of not more than ppm to give a highly purified bomb, and there is a problem that the concentration of residual impurity gases which are not dissolved in a solvent such as nitrogen gas and oxygen gas remained in the space, the mass and the like of the bomb is increased to about 100 to 300 ppm when the bomb is allowed to stand for several days although the inside of the bomb is purified to give acetylene having an apparent purity of 5N (five nine), that is, 99.999% by volume in the middle course of liberating when the inside of the bomb is cleaned with acetylene repeatedly.
As mentioned above, a dissolved acetylene bomb which can accommodate and supply high purity acetylene with keeping the purity has not yet been developed. Accordingly, the development of a bomb for high purity dissolved acetylene has been eagered in the present technical field.
Third, the object of the present invention is to provide a bomb for high purity dissolved acetylene.
Also, according to a conventional process for charging acetylene gas into a bomb, there are following problems.
(a) Since acetylene is a chemically unstable compound and there is a risk to generate explosive decomposition when acetylene is abruptly pressured, the acetylene is pressured so that the compression ratio is not more than 3. However, it takes a long time, that is, 6 to 8 hours for charging, e.g., a prescribed amount of acetylene (7.2 Kg) into a bomb (content volume:41 l) which is usually employed, and further, since the final pressure when acetylene is charged into a bomb is high, that is, 20 to 25 Kg/cm.sup.2.G, acetylene is in extremely unstable and dangerous state.
(b) In case that acetylene gas is charged into a bomb, since a compressor is employed, there is a necessity to use a series of equipment such as an oil separator, a high pressure drier, and the like. Further, there are problems that it is difficult to replace residual air in the series of the equipment with acetylene which is beforehand purified, that a large amount of acetylene gas for replacing is needed, that it takes a long time for replacing and that it is difficult to keep the purity of the equipment.
Fourth, the object of the present invention is to provide a process for filling a bomb with high purity acetylene gas to solve the above problems.